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rfc:rfc4348

Network Working Group S. Ahmadi Request for Comments: 4348 January 2006 Category: Standards Track

     Real-Time Transport Protocol (RTP) Payload Format for the
       Variable-Rate Multimode Wideband (VMR-WB) Audio Codec

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This document specifies a real-time transport protocol (RTP) payload
 format to be used for the Variable-Rate Multimode Wideband (VMR-WB)
 speech codec.  The payload format is designed to be able to
 interoperate with existing VMR-WB transport formats on non-IP
 networks.  A media type registration is included for VMR-WB RTP
 payload format.
 VMR-WB is a variable-rate multimode wideband speech codec that has a
 number of operating modes, one of which is interoperable with AMR-WB
 (i.e., RFC 3267) audio codec at certain rates.  Therefore, provisions
 have been made in this document to facilitate and simplify data
 packet exchange between VMR-WB and AMR-WB in the interoperable mode
 with no transcoding function involved.

Ahmadi Standards Track [Page 1] RFC 4348 VMR-WB RTP Payload Format January 2006

Table of Contents

 1. Introduction ....................................................3
 2. Conventions and Acronyms ........................................3
 3. The Variable-Rate Multimode Wideband (VMR-WB) Speech Codec ......4
    3.1. Narrowband Speech Processing ...............................5
    3.2. Continuous vs. Discontinuous Transmission ..................6
    3.3. Support for Multi-Channel Session ..........................6
 4. Robustness against Packet Loss ..................................7
    4.1. Forward Error Correction (FEC) .............................7
    4.2. Frame Interleaving and Multi-Frame Encapsulation ...........8
 5. VMR-WB Voice over IP Scenarios ..................................9
    5.1. IP Terminal to IP Terminal .................................9
    5.2. GW to IP Terminal .........................................10
    5.3. GW to GW (between VMR-WB- and AMR-WB-Enabled Terminals) ...10
    5.4. GW to GW (between Two VMR-WB-Enabled Terminals) ...........11
 6. VMR-WB RTP Payload Formats .....................................12
    6.1. RTP Header Usage ..........................................13
    6.2. Header-Free Payload Format ................................14
    6.3. Octet-Aligned Payload Format ..............................15
         6.3.1. Payload Structure ..................................15
         6.3.2. The Payload Header .................................15
         6.3.3. The Payload Table of Contents ......................18
         6.3.4. Speech Data ........................................20
         6.3.5. Payload Example: Basic Single Channel
                Payload Carrying Multiple Frames ...................21
    6.4. Implementation Considerations .............................22
         6.4.1. Decoding Validation and Provision for Lost
                or Late Packets ....................................22
 7. Congestion Control .............................................23
 8. Security Considerations ........................................23
    8.1. Confidentiality ...........................................24
    8.2. Authentication and Integrity ..............................24
 9. Payload Format Parameters ......................................24
    9.1. VMR-WB RTP Payload MIME Registration ......................25
    9.2. Mapping MIME Parameters into SDP ..........................27
    9.3. Offer-Answer Model Considerations .........................28
 10. IANA Considerations ...........................................29
 11. Acknowledgements ..............................................29
 12. References ....................................................30
    12.1. Normative References .....................................30
    12.2. Informative References ...................................30

Ahmadi Standards Track [Page 2] RFC 4348 VMR-WB RTP Payload Format January 2006

1. Introduction

 This document specifies the payload format for packetization of VMR-
 WB-encoded speech signals into the Real-time Transport Protocol (RTP)
 [3].  The VMR-WB payload formats support transmission of single and
 multiple channels, frame interleaving, multiple frames per payload,
 header-free payload, the use of mode switching, and interoperation
 with existing VMR-WB transport formats on non-IP networks, as
 described in Section 3.
 The payload format is described in Section 6.  The VMR-WB file format
 (i.e., for transport of VMR-WB speech data in storage mode
 applications such as email) is specified in [7].  In Section 9, a
 media type registration for VMR-WB RTP payload format is provided.
 Since VMR-WB is interoperable with AMR-WB at certain rates, an
 attempt has been made throughout this document to maximize the
 similarities with RFC 3267 while optimizing the payload format for
 the non-interoperable modes of the VMR-WB codec.

2. Conventions and Acronyms

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC2119 [2].
 The following acronyms are used in this document:
  3GPP   - The Third Generation Partnership Project
  3GPP2  - The Third Generation Partnership Project 2
  CDMA   - Code Division Multiple Access
  WCDMA  - Wideband Code Division Multiple Access
  GSM    - Global System for Mobile Communications
  AMR-WB - Adaptive Multi-Rate Wideband Codec
  VMR-WB - Variable-Rate Multimode Wideband Codec
  CMR    - Codec Mode Request
  GW     - Gateway
  DTX    - Discontinuous Transmission
  FEC    - Forward Error Correction
  SID    - Silence Descriptor
  TrFO   - Transcoder-Free Operation
  UDP    - User Datagram Protocol
  RTP    - Real-Time Transport Protocol
  RTCP   - RTP Control Protocol
  MIME   - Multipurpose Internet Mail Extension
  SDP    - Session Description Protocol
  VoIP   - Voice-over-IP

Ahmadi Standards Track [Page 3] RFC 4348 VMR-WB RTP Payload Format January 2006

 The term "interoperable mode" in this document refers to VMR-WB mode
 3, which is interoperable with AMR-WB codec modes 0, 1, and 2.
 The term "non-interoperable modes" in this document refers to VMR-WB
 modes 0, 1, and 2.
 The term "frame-block" is used in this document to describe the
 time-synchronized set of speech frames in a multi-channel VMR-WB
 session.  In particular, in an N-channel session, a frame-block will
 contain N speech frames, one from each of the channels, and all N
 speech frames represent exactly the same time period.

3. The Variable-Rate Multimode Wideband (VMR-WB) Speech Codec

 VMR-WB is the wideband speech-coding standard developed by Third
 Generation Partnership Project 2 (3GPP2) for encoding/decoding
 wideband/narrowband speech content in multimedia services in 3G CDMA
 cellular systems [1].  VMR-WB is a source-controlled variable-rate
 multimode wideband speech codec.  It has a number of operating modes,
 where each mode is a tradeoff between voice quality and average data
 rate.  The operating mode in VMR-WB (as shown in Table 2) is chosen
 based on the traffic condition of the network and the desired quality
 of service.  The desired average data rate (ADR) in each mode is
 obtained by encoding speech frames at permissible rates (as shown in
 Tables 1 and 3) compliant with CDMA2000 system, depending on the
 instantaneous characteristics of input speech and the maximum and
 minimum rate constraints imposed by the network operator.
 While VMR-WB is a native CDMA codec complying with all CDMA system
 requirements, it is further interoperable with AMR-WB [4,12] at
 12.65, 8.85, and 6.60 kbps.  This is due to the fact that VMR-WB and
 AMR-WB share the same core technology.  This feature enables
 Transcoder-Free (TrFO) interconnections between VMR-WB and AMR-WB
 across different wireless/wireline systems (e.g., GSM/WCDMA and
 CDMA2000) without use of unnecessary complex media format conversion.
 Note that the concept of mode in VMR-WB is different from that of
 AMR-WB where each fixed-rate AMR-WB codec mode is adapted to
 prevailing channel conditions by a tradeoff between the total number
 of source-coding and channel-coding bits.
 VMR-WB is able to transition between various modes with no
 degradation in voice quality that is attributable to the mode
 switching itself.  The operating mode of the VMR-WB encoder may be
 switched seamlessly without prior knowledge of the decoder.  Any
 non-interoperable mode (i.e., VMR-WB modes 0, 1, or 2) can be chosen
 depending on the traffic conditions (e.g., network congestion) and
 the desired quality of service.

Ahmadi Standards Track [Page 4] RFC 4348 VMR-WB RTP Payload Format January 2006

 While in the interoperable mode (i.e., VMR-WB mode 3), mode switching
 between VMR-WB modes is not allowed because there is only one AMR-WB
 interoperable mode in VMR-WB.  Since the AMR-WB codec may request a
 mode change, depending on channel conditions, in-band data included
 in VMR-WB frame structure (see Section 8 of [1] for more details) is
 used during an interoperable interconnection to switch between VMR-WB
 frame types 0, 1, and 2 in VMR-WB mode 3 (corresponding to AMR-WB
 codec modes 0, 1, or 2).
 As mentioned earlier, VMR-WB is compliant with CDMA2000 system with
 the permissible encoding rates shown in Table 1.
 +---------------------------+-----------------+---------------+
 |        Frame Type         | Bits per Packet | Encoding Rate |
 |                           |   (Frame Size)  |     (kbps)    |
 +---------------------------+-----------------+---------------+
 | Full-Rate                 |      266        |     13.3      |
 | Half-Rate                 |      124        |      6.2      |
 | Quarter-Rate              |       54        |      2.7      |
 | Eighth-Rate               |       20        |      1.0      |
 | Blank                     |        0        |       0       |
 | Erasure                   |        0        |       0       |
 +---------------------------+-----------------+---------------+
   Table 1: CDMA2000 system permissible frame types and their
            associated encoding rates
 VMR-WB is robust to high percentage of frame loss and frames with
 corrupted rate information.  The reception of an Erasure
 (SPEECH_LOST) frame type at decoder invokes the built-in frame error
 concealment mechanism.  The built-in frame error concealment
 mechanism in VMR-WB conceals the effect of lost frames by exploiting
 in-band data and the information available in the previous frames.

3.1. Narrowband Speech Processing

 VMR-WB has the capability to operate with either 16000-Hz or 8000-Hz
 sampled input/output speech signals in all modes of operation [1].
 The VMR-WB decoder does not require a priori knowledge about the
 sampling rate of the original media (i.e., speech/audio signals
 sampled at 8 or 16 kHz) at the input of the encoder.  The VMR-WB
 decoder, by default, generates 16000-Hz wideband output regardless of
 the encoder input sampling frequency.  Depending on the application,
 the decoder can be configured to generate 8000-Hz output, as well.

Ahmadi Standards Track [Page 5] RFC 4348 VMR-WB RTP Payload Format January 2006

 Therefore, while this specification defines a 16000-Hz RTP clock rate
 for VMR-WB codec, the injection and processing of 8000-Hz narrowband
 media during a session is also allowed; however, a 16000-Hz RTP clock
 rate MUST always be used.
 The choice of VMR-WB output sampling frequency depends on the
 implementation and the audio acoustic capabilities of the receiving
 side.

3.2. Continuous vs. Discontinuous Transmission

 The circuit-switched operation of VMR-WB within a CDMA network
 requires continuous transmission of the speech data during a
 conversation.  The intrinsic source-controlled variable-rate feature
 of the CDMA speech codecs is required for optimal operation of the
 CDMA system and interference control.  However, VMR-WB has the
 capability to operate in a discontinuous transmission mode for some
 packet-switched applications over IP networks (e.g., VoIP), where the
 number of transmitted bits and packets during silence period are
 reduced to a minimum.  The VMR-WB DTX operation is similar to that of
 AMR-WB [4,12].

3.3. Support for Multi-Channel Session

 The octet-aligned RTP payload format defined in this document
 supports multi-channel audio content (e.g., a stereophonic speech
 session).  Although VMR-WB codec itself does not support encoding of
 multi-channel audio content into a single bit stream, it can be used
 to encode and decode each of the individual channels separately.
 To transport the separately encoded multi-channel content, the speech
 frames for all channels that are framed and encoded for the same 20
 ms periods are logically collected in a frame-block.
 At the session setup, out-of-band signaling must be used to indicate
 the number of channels in the session and the order of the speech
 frames from different channels in each frame-block.  When using SDP
 for signaling (see Section 9.2 for more details), the number of
 channels is specified in the rtpmap attribute, and the order of
 channels carried in each frame-block is implied by the number of
 channels as specified in Section 4.1 in [6].

Ahmadi Standards Track [Page 6] RFC 4348 VMR-WB RTP Payload Format January 2006

4. Robustness against Packet Loss

 The octet-aligned payload format described in this document (see
 Section 6 for more details) supports several features, including
 forward error correction (FEC) and frame interleaving, in order to
 increase robustness against lost packets.

4.1. Forward Error Correction (FEC)

 The simple scheme of repetition of previously sent data is one way of
 achieving FEC.  Another possible scheme, which is more bandwidth
 efficient, is to use payload-external FEC; e.g., RFC2733 [8], which
 generates extra packets containing repair data.
 The repetition method involves the simple retransmission of
 previously transmitted frame-blocks together with the current frame-
 block(s).  This is done by using a sliding window to group the speech
 frame-blocks to send in each payload.  Figure 1 illustrates an
 example.
 In this example, each frame-block is retransmitted one time in the
 following RTP payload packet.  Here, f(n-2)..f(n+4) denotes a
 sequence of speech frame-blocks, and p(n-1)..p(n+4) a sequence of
 payload packets.
  1. -+——–+——–+——–+——–+——–+——–+——–+–

| f(n-2) | f(n-1) | f(n) | f(n+1) | f(n+2) | f(n+3) | f(n+4) |

  1. -+——–+——–+——–+——–+——–+——–+——–+–
   <---- p(n-1) ---->
            <----- p(n) ----->
                     <---- p(n+1) ---->
                              <---- p(n+2) ---->
                                       <---- p(n+3) ---->
                                                <---- p(n+4) ---->
            Figure 1: An example of redundant transmission
 The use of this approach does not require signaling at the session
 setup.  In other words, the speech sender can choose to use this
 scheme without consulting the receiver.  This is because a packet
 containing redundant frames will not look different from a packet
 with only new frames.  The receiver may receive multiple copies or
 versions of a frame for a certain timestamp if no packet is lost.  If
 multiple versions of the same speech frame are received, it is
 RECOMMENDED that the highest rate be used by the speech decoder.

Ahmadi Standards Track [Page 7] RFC 4348 VMR-WB RTP Payload Format January 2006

 This redundancy scheme provides the same functionality as that
 described in RFC 2198, "RTP Payload for Redundant Audio Data" [10].
 In most cases, the mechanism in this payload format is more efficient
 and simpler than requiring both endpoints to support RFC 2198.  If
 the spread in time required between the primary and redundant
 encodings is larger than 5 frame times, the bandwidth overhead of RFC
 2198 will be lower.
 The sender is responsible for selecting an appropriate amount of
 redundancy based on feedback about the channel (e.g., in RTCP
 receiver reports) or network traffic.  A sender SHOULD NOT base
 selection of FEC on the CMR, as this parameter most probably was set
 based on non-IP information.  The sender is also responsible for
 avoiding congestion, which may be aggravated by redundant
 transmission (see Section 7).

4.2. Frame Interleaving and Multi-Frame Encapsulation

 To decrease protocol overhead, the octet-aligned payload format,
 described in Section 6, allows several speech frame-blocks to be
 encapsulated into a single RTP packet.  One of the drawbacks of this
 approach is that in case of packet loss several consecutive speech
 frame-blocks are lost, which usually causes clearly audible
 distortion in the reconstructed speech.
 Interleaving of frame-blocks can improve the speech quality in such
 cases by distributing the consecutive losses into a series of single
 frame-block losses.  However, interleaving and bundling several
 frame-blocks per payload will also increase end-to-end delay and is
 therefore not appropriate for all types of applications.  Streaming
 applications will most likely be able to exploit interleaving to
 improve speech quality in lossy transmission conditions.
 The octet-aligned payload format supports the use of frame
 interleaving as an option.  For the encoder (speech sender) to use
 frame interleaving in its outbound RTP packets for a given session,
 the decoder (speech receiver) needs to indicate its support via out-
 of-band means (see Section 9).

Ahmadi Standards Track [Page 8] RFC 4348 VMR-WB RTP Payload Format January 2006

5. VMR-WB Voice over IP Scenarios

5.1. IP Terminal to IP Terminal

 The primary scenario for this payload format is IP end-to-end between
 two terminals incorporating VMR-WB codec, as shown in Figure 2.
 Nevertheless, this scenario can be generalized to an interoperable
 interconnection between VMR-WB-enabled and AMR-WB-enabled IP
 terminals using the offer-answer model described in Section 9.3.
 This payload format is expected to be useful for both conversational
 and streaming services.
     +----------+                         +----------+
     |          |                         |          |
     | TERMINAL |<----------------------->| TERMINAL |
     |          |    VMR-WB/RTP/UDP/IP    |          |
     +----------+                         +----------+
                   (or AMR-WB/RTP/UDP/IP)
        Figure 2: IP terminal to IP terminal
 A conversational service puts requirements on the payload format.
 Low delay is a very important factor, i.e., fewer speech frame-blocks
 per payload packet.  Low overhead is also required when the payload
 format traverses across low bandwidth links, especially if the
 frequency of packets will be high.
 Streaming service has less strict real-time requirements and
 therefore can use a larger number of frame-blocks per packet than
 conversational service.  This reduces the overhead from IP, UDP, and
 RTP headers.  However, including several frame-blocks per packet
 makes the transmission more vulnerable to packet loss, so
 interleaving may be used to reduce the effect of packet loss on
 speech quality.  A streaming server handling a large number of
 clients also needs a payload format that requires as few resources as
 possible when doing packetization.
 For VMR-WB-enabled IP terminals at both ends, depending on the
 implementation, all modes of the VMR-WB codec can be used in this
 scenario.  Also, both header-free and octet-aligned payload formats
 (see Section 6 for details) can be utilized.  For the interoperable
 interconnection between VMR-WB and AMR-WB, only VMR-WB mode 3 is
 used, and all restrictions described in Section 9.3 apply.

Ahmadi Standards Track [Page 9] RFC 4348 VMR-WB RTP Payload Format January 2006

5.2. GW to IP Terminal

 Another scenario occurs when VMR-WB-encoded speech will be
 transmitted from a non-IP system (e.g., 3GPP2/CDMA2000 network) to an
 IP terminal, and/or vice versa, as depicted in Figure 3.
     VMR-WB over
 3GPP2/CDMA2000 network
                    +------+                        +----------+
                    |      |                        |          |
    <-------------->|  GW  |<---------------------->| TERMINAL |
                    |      |   VMR-WB/RTP/UDP/IP    |          |
                    +------+                        +----------+
                        |
                        |           IP network
                        |
                 Figure 3: GW to VoIP terminal scenario
 VMR-WB's capability to switch seamlessly between operational modes is
 exploited in CDMA (non-IP) networks to optimize speech quality for a
 given traffic condition.  To preserve this functionality in scenarios
 including a gateway to an IP network using the octet-aligned payload
 format, a codec mode request (CMR) field is considered.  The gateway
 will be responsible for forwarding the CMR between the non-IP and IP
 parts in both directions.  The IP terminal SHOULD follow the CMR
 forwarded by the gateway to optimize speech quality going to the
 non-IP decoder.  The mode control algorithm in the gateway SHOULD
 accommodate the delay imposed by the IP network on the response to
 CMR by the IP terminal.
 The IP terminal SHOULD NOT set the CMR (see Section 6.3.2), but the
 gateway can set the CMR value on frames going toward the encoder in
 the non-IP part to optimize speech quality from that encoder to the
 gateway and to perform congestion control on the IP network.

5.3. GW to GW (between VMR-WB- and AMR-WB-Enabled Terminals)

 A third likely scenario is that RTP/UDP/IP is used as transport
 between two non-IP systems, i.e., IP is originated and terminated in
 gateways on both sides of the IP transport, as illustrated in Figure
 4.  This is the most likely scenario for an interoperable
 interconnection between 3GPP/(GSM-WCDMA)/AMR-WB and
 3GPP2/CDMA2000/VMR-WB-enabled mobile stations.  In this scenario, the
 VMR-WB-enabled terminal also declares itself capable of AMR-WB with
 restricted mode set as described in Section 9.3. The CMR value may be
 set in packets received by the gateways on the IP network side.  The
 gateway should forward to the non-IP side a CMR value that is the

Ahmadi Standards Track [Page 10] RFC 4348 VMR-WB RTP Payload Format January 2006

 minimum of three values: (1) the CMR value it receives on the IP
 side; (2) a CMR value it may choose for congestion control of
 transmission on the IP side; and (3) the CMR value based on its
 estimate of reception quality on the non-IP side.  The details of the
 traffic control algorithm are left to the implementation.
    VMR-WB over                                       AMR-WB over
 3GPP2/CDMA2000 network                      3GPP/(GSM-WCDMA) network
                   +------+                  +------+
  (AMR-WB Payload) |      | AMR-WB/RTP/UDP/IP|      |(AMR-WB Payload)
 <---------------->|  GW  |<---------------->|  GW  |<--------------->
                   |      |                  |      |
                   +------+                  +------+
                      |        IP network       |
                      |                         |
             Figure 4: GW to GW scenario (AMR-WB <-> VMR-WB
                    interoperable interconnection)
 During and upon initiation of an interoperable interconnection
 between VMR-WB and AMR-WB, only VMR-WB mode 3 can be used.  There are
 three Frame Types (i.e., FT=0, 1, or 2; see Table 3) within this mode
 that are compatible with AMR-WB codec modes 0, 1, and 2,
 respectively.  If the AMR-WB codec is engaged in an interoperable
 interconnection with VMR-WB, the active AMR-WB codec mode set needs
 to be limited to 0, 1, and 2.

5.4. GW to GW (between Two VMR-WB-Enabled Terminals)

 The fourth example VoIP scenario is composed of a RTP/UDP/IP
 transport between two non-IP systems; i.e., IP is originated and
 terminated in gateways on both sides of the IP transport, as
 illustrated in Figure 5.  This is the most likely scenario for
 Mobile-Station-to-Mobile-Station (MS-to-MS) Transcoder-Free (TrFO)
 interconnection between two 3GPP2/CDMA2000 terminals that both use
 VMR-WB codec.

Ahmadi Standards Track [Page 11] RFC 4348 VMR-WB RTP Payload Format January 2006

      VMR-WB over                                     VMR-WB over
 3GPP2/CDMA2000 network                         3GPP2/CDMA2000 network
                    +------+                   +------+
                    |      |                   |      |
      <------------>|  GW  |<----------------->|  GW  |<------------>
                    |      | VMR-WB/RTP/UDP/IP |      |
                    +------+                   +------+
                        |         IP network       |
                        |                          |
      Figure 5: GW to GW scenario (a CDMA2000 MS-to-MS VoIP scenario)

6. VMR-WB RTP Payload Formats

 For a given session, the payload format can be either header free or
 octet aligned, depending on the mode of operation that is established
 for the session via out-of-band means and the application.
 The header-free payload format is designed for maximum bandwidth
 efficiency, simplicity, and low latency.  Only one codec data frame
 can be sent in each header-free payload format packet.  None of the
 payload header fields or table of contents (ToC) entries is present
 (the same consideration is also made in [11]).
 In the octet-aligned payload format, all the fields in a payload,
 including payload header, table of contents entries, and speech
 frames themselves, are individually aligned to octet boundaries to
 make implementations efficient.
 Note that octet alignment of a field or payload means that the last
 octet is padded with zeroes in the least significant bits to fill the
 octet.  Also note that this padding is separate from padding
 indicated by the P bit in the RTP header.
 Between the two payload formats, only the octet-aligned format has
 the capability to use the interleaving to make the speech transport
 robust to packet loss.
 The VMR-WB octet-aligned payload format in the interoperable mode is
 identical to that of AMR-WB (i.e., RFC 3267).

Ahmadi Standards Track [Page 12] RFC 4348 VMR-WB RTP Payload Format January 2006

6.1. RTP Header Usage

 The format of the RTP header is specified in [3].  This payload
 format uses the fields of the header in a manner consistent with that
 specification.
 The RTP timestamp corresponds to the sampling instant of the first
 sample encoded for the first frame-block in the packet.  The
 timestamp clock frequency is the same as the default sampling
 frequency (i.e., 16 kHz), so the timestamp unit is in samples.
 The duration of one speech frame-block is 20 ms for VMR-WB.  For
 normal wideband operation of VMR-WB, the input/output media sampling
 frequency is 16 kHz, corresponding to 320 samples per frame from each
 channel.  Thus, the timestamp is increased by 320 for VMR-WB for each
 consecutive frame-block.
 The VMR-WB codec is capable of processing speech/audio signals
 sampled at 8 kHz.  By default, the VMR-WB decoder output sampling
 frequency is 16 kHz.  Depending on the application, the decoder can
 be configured to generate 8-kHz output sampling frequency, as well.
 Since the VMR-WB RTP payload formats for the 8- and 16-kHz sampled
 media are identical and the VMR-WB decoder does not need a priori
 knowledge about the encoder input sampling frequency, a fixed RTP
 clock rate of 16000 Hz is defined for VMR-WB codec.  This would allow
 injection or processing of 8-kHz sampled speech/audio media without
 having to change the RTP clock rate during a session.  Note that the
 timestamp is incremented by 320 per frame-block for 8-kHz sampled
 media, as well.
 A packet may contain multiple frame-blocks of encoded speech or
 comfort noise parameters.  If interleaving is employed, the frame-
 blocks encapsulated into a payload are picked according to the
 interleaving rules defined in Section 6.3.2. Otherwise, each packet
 covers a period of one or more contiguous 20-ms frame-block
 intervals.  In case the data from all the channels for a particular
 frame-block in the period is missing (for example, at a gateway from
 some other transport format), it is possible to indicate that no data
 is present for that frame-block instead of breaking a multi-frame-
 block packet into two, as explained in Section 6.3.2.
 No matter which payload format is used, the RTP payload is always
 made an integral number of octets long by padding with zero bits if
 necessary.  If additional padding is required to bring the payload
 length to a larger multiple of octets or for some other purpose, then
 the P bit in the RTP header MAY be set, and padding appended, as
 specified in [3].

Ahmadi Standards Track [Page 13] RFC 4348 VMR-WB RTP Payload Format January 2006

 The RTP header marker bit (M) SHALL be always set to 0 if the VMR-WB
 codec operates in continuous transmission.  When operating in
 discontinuous transmission (DTX), the RTP header marker bit SHALL be
 set to 1 if the first frame-block carried in the packet contains a
 speech frame, which is the first in a talkspurt.  For all other
 packets, the marker bit SHALL be set to zero (M=0).
 The assignment of an RTP payload type for this payload format is
 outside the scope of this document and will not be specified here.
 It is expected that the RTP profile under which this payload format
 is being used will assign a payload type for this encoding or specify
 that the payload type is to be bound dynamically (see Section 9).

6.2. Header-Free Payload Format

 The header-free payload format is designed for maximum bandwidth
 efficiency, simplicity, and minimum delay.  Only one speech data
 frame presents in each header-free payload format packet.  None of
 the payload header fields or ToC entries is present.  The encoding
 rate for the speech frame can be determined from the length of the
 speech data frame, since there is only one speech data frame in each
 header-free payload format.
 The use of the RTP header fields for header-free payload format is
 the same as the corresponding one for the octet-aligned payload
 format.  The detailed bit mapping of speech data packets permissible
 for this payload format is described in Section 8 of [1].  Since the
 header-free payload format is not compatible with AMR-WB RTP payload,
 only non-interoperable modes of VMR-WB SHALL be used with this
 payload format.  That is, FT=0, 1, 2, and 9 SHALL NOT be used with
 header-free payload format.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      RTP Header [3]                           |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                                                               |
 +          ONLY one speech data frame           +-+-+-+-+-+-+-+-+
 |                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Note that the mode of operation, using this payload format, is
 decided by the transmitting (encoder) site.  The default mode of
 operation for VMR-WB encoder is mode 0 [1].  The mode change request
 MAY also be sent through non-RTP means, which is out of the scope of
 this specification.

Ahmadi Standards Track [Page 14] RFC 4348 VMR-WB RTP Payload Format January 2006

6.3. Octet-Aligned Payload Format

6.3.1. Payload Structure

 The complete payload consists of a payload header, a payload table of
 contents, and speech data representing one or more speech frame-
 blocks.  The following diagram shows the general payload format
 layout:
 +----------------+-------------------+----------------
 | Payload header | Table of contents | Speech data ...
 +----------------+-------------------+----------------

6.3.2. The Payload Header

 In octet-aligned payload format, the payload header consists of a
 4-bit CMR, 4 reserved bits, and, optionally, an 8-bit interleaving
 header, as shown below.
  0                   1
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 +-+-+-+-+-+-+-+-+- - - - - - - -
 |  CMR  |R|R|R|R|  ILL  |  ILP  |
 +-+-+-+-+-+-+-+-+- - - - - - - -
 CMR (4 bits): This indicates a codec mode request sent to the speech
 encoder at the site of the receiver of this payload.  CMR value 15
 indicates that no mode request is present, and other unused values
 are reserved for future use.
 The value of the CMR field is set according to the following table:
 +-------+----------------------------------------------------------+
 | CMR   |                 VMR-WB Operating Modes                   |
 +-------+----------------------------------------------------------+
 |   0   | VMR-WB mode 3 (AMR-WB interoperable mode at 6.60 kbps)   |
 |   1   | VMR-WB mode 3 (AMR-WB interoperable mode at 8.85 kbps)   |
 |   2   | VMR-WB mode 3 (AMR-WB interoperable mode at 12.65 kbps)  |
 |   3   | VMR-WB mode 2                                            |
 |   4   | VMR-WB mode 1                                            |
 |   5   | VMR-WB mode 0                                            |
 |   6   | VMR-WB mode 2 with maximum half-rate encoding            |
 | 7-14  | (reserved)                                               |
 |  15   | No Preference (no mode request is present)               |
 +-------+----------------------------------------------------------+
   Table 2: List of valid CMR values and their associated VMR-WB
            operating modes

Ahmadi Standards Track [Page 15] RFC 4348 VMR-WB RTP Payload Format January 2006

 R: This is a reserved bit that MUST be set to zero.  The receiver
 MUST ignore all R bits.
 ILL (4 bits, unsigned integer): This is an OPTIONAL field that is
 present only if interleaving is signaled out-of-band for the session.
 ILL=L indicates to the receiver that the interleaving length is L+1,
 in number of frame-blocks.
 ILP (4 bits, unsigned integer): This is an OPTIONAL field that is
 present only if interleaving is signaled.  ILP MUST take a value
 between 0 and ILL, inclusive, indicating the interleaving index for
 frame-blocks in this payload in the interleave group.  If the value
 of ILP is found greater than ILL, the payload SHOULD be discarded.
 ILL and ILP fields MUST be present in each packet in a session if
 interleaving is signaled for the session.
 The mode request received in the CMR field is valid until the next
 CMR is received, i.e., until a newly received CMR value overrides the
 previous one.  Therefore, if a terminal continuously wishes to
 receive frames in the same mode, x, it needs to set CMR=x for all its
 outbound payloads, and if a terminal has no preference in which mode
 to receive, it SHOULD set CMR=15 in all its outbound payloads.
 If a payload is received with a CMR value that is not valid, the CMR
 MUST be ignored by the receiver.
 In a multi-channel session, CMR SHOULD be interpreted by the receiver
 of the payload as the desired encoding mode for all the channels in
 the session, if the network allows.
 There are two factors that affect the VMR-WB mode selection: (i) the
 performance of any CDMA link connected via a gateway (e.g., in a GW
 to IP terminal scenario), and (ii) the congestion state of an IP
 network.  The CDMA link performance is signaled via the CMR field,
 which is not used by IP-only end-points.  The IP network state is
 monitored using, for example, RTCP.  A sender needs to select the
 operating mode to satisfy both these constraints (see Section 7).
 The encoder SHOULD follow a received mode request, but MAY change to
 a different mode if the network necessitates it, for example, to
 control congestion.
 The CMR field MUST be set to 15 for packets sent to a multicast
 group.  The encoder in the speech sender SHOULD ignore mode requests
 when sending speech to a multicast session but MAY use RTCP feedback
 information as a hint that a mode change is needed.

Ahmadi Standards Track [Page 16] RFC 4348 VMR-WB RTP Payload Format January 2006

 If interleaving option is utilized, interleaving MUST be performed on
 a frame-block basis, as opposed to a frame basis, in a multi-channel
 session.
 The following example illustrates the arrangement of speech frame-
 blocks in an interleave group during an interleave session.  Here we
 assume ILL=L for the interleave group that starts at speech frame-
 block n.  We also assume that the first payload packet of the
 interleave group is s and the number of speech frame-blocks carried
 in each payload is N.  Then we will have
  Payload s (the first packet of this interleave group):
    ILL=L, ILP=0,
  Carry frame-blocks: n, n+(L+1), n+2*(L+1),..., n+(N-1)*(L+1)
  Payload s+1 (the second packet of this interleave group):
    ILL=L, ILP=1,
    Carry frame-blocks: n+1, n+1+(L+1), n+1+2*(L+1),..., n+1+
    (N-1)*(L+1)
      ...
  Payload s+L (the last packet of this interleave group):
    ILL=L, ILP=L,
    Carry frame-blocks: n+L, n+L+(L+1), n+L+2*(L+1), ..., n+L+
    (N-1)*(L+1)
 The next interleave group will start at frame-block n+N*(L+1).  There
 will be no interleaving effect unless the number of frame-blocks per
 packet (N) is at least 2.  Moreover, the number of frame-blocks per
 payload (N) and the value of ILL MUST NOT be changed inside an
 interleave group.  In other words, all payloads in an interleave
 group MUST have the same ILL and MUST contain the same number of
 speech frame-blocks.
 The sender of the payload MUST only apply interleaving if the
 receiver has signaled its use through out-of-band means.  Since
 interleaving will increase buffering requirements at the receiver,
 the receiver uses MIME parameter "interleaving=I" to set the maximum
 number of frame-blocks allowed in an interleaving group to I.
 When performing interleaving, the sender MUST use a proper number of
 frame-blocks per payload (N) and ILL so that the resulting size of an
 interleave group is less than or equal to I, i.e., N*(L+1)<=I.
 The following example shows the ToC of three consecutive packets,
 each carrying 3 frame-blocks, in an interleaved two-channel session.

Ahmadi Standards Track [Page 17] RFC 4348 VMR-WB RTP Payload Format January 2006

 Here, the two channels are left (L) and right (R), with L coming
 before R, and the interleaving length is 3 (i.e., ILL=2).  This makes
 the interleave group 9 frame-blocks large.
 Packet #1
 ---------
 ILL=2, ILP=0:
 +----+----+----+----+----+----+
 | 1L | 1R | 4L | 4R | 7L | 7R |
 +----+----+----+----+----+----+
 |<------->|<------->|<------->|
    Frame     Frame     Frame
   Block 1   Block 4   Block 7
 Packet #2
 ---------
 ILL=2, ILP=1:
 +----+----+----+----+----+----+
 | 2L | 2R | 5L | 5R | 8L | 8R |
 +----+----+----+----+----+----+
 |<------->|<------->|<------->|
    Frame     Frame     Frame
   Block 2   Block 5   Block 8
 Packet #3
 ---------
 ILL=2, ILP=2:
 +----+----+----+----+----+----+
 | 3L | 3R | 6L | 6R | 9L | 9R |
 +----+----+----+----+----+----+
 |<------->|<------->|<------->|
       Frame     Frame     Frame
      Block 3   Block 6   Block 9

6.3.3. The Payload Table of Contents

 The table of contents (ToC) in octet-aligned payload format consists
 of a list of ToC entries where each entry corresponds to a speech
 frame carried in the payload, i.e., when interleaving is used, the
 frame-blocks in the ToC will almost never be placed consecutive in
 time.  Instead, the presence and order of the frame-blocks in a
 packet will follow the pattern described in 6.3.2.

Ahmadi Standards Track [Page 18] RFC 4348 VMR-WB RTP Payload Format January 2006

 +---------------------+
 | list of ToC entries |
 +---------------------+
 A ToC entry for the octet-aligned payload format is as follows:
  0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |F|  FT   |Q|P|P|
 +-+-+-+-+-+-+-+-+
 The table of contents (ToC) consists of a list of ToC entries, each
 representing a speech frame.
 F (1 bit):   If set to 1, indicates that this frame is followed by
              another speech frame in this payload; if set to 0,
              indicates that this frame is the last frame in this
              payload.
 FT (4 bits): Frame type index whose value is chosen according to
              Table 3.
              During the interoperable mode, FT=14 (SPEECH_LOST) and
              FT=15 (NO_DATA) are used to indicate frames that are
              either lost or not being transmitted in this payload,
              respectively.  FT=14 or 15 MAY be used in the non-
              interoperable modes to indicate frame erasure or blank
              frame, respectively (see Section 2.1 of [1]).
              If a payload with an invalid FT value is received, the
              payload MUST be discarded.  Note that for ToC entries
              with FT=14 or 15, there will be no corresponding speech
              frame in the payload.
              Depending on the application and the mode of operation
              of VMR-WB, any combination of the permissible frame
              types (FT) shown in Table 3 MAY be used.
 Q (1 bit):   Frame quality indicator.  If set to 0, indicates that
              the corresponding frame is corrupted.  During the
              interoperable mode, the receiver side (with AMR-WB
              codec) should set the RX_TYPE to either SPEECH_BAD or
              SID_BAD depending on the frame type (FT), if Q=0.  The
              VMR-WB encoder always sets Q bit to 1.  The VMR-WB
              decoder may ignore the Q bit.
 P bits:      Padding bits MUST be set to zero and MUST be ignored by
              a receiver.

Ahmadi Standards Track [Page 19] RFC 4348 VMR-WB RTP Payload Format January 2006

 +----+--------------------------------------------+-----------------+
 | FT |                Encoding Rate               |Frame Size (Bits)|
 +----+--------------------------------------------+-----------------+
 | 0  | Interoperable Full-Rate (AMR-WB 6.60 kbps) |       132       |
 | 1  | Interoperable Full-Rate (AMR-WB 8.85 kbps) |       177       |
 | 2  | Interoperable Full-Rate (AMR-WB 12.65 kbps)|       253       |
 | 3  | Full-Rate 13.3 kbps                        |       266       |
 | 4  | Half-Rate 6.2 kbps                         |       124       |
 | 5  | Quarter-Rate 2.7 kbps                      |        54       |
 | 6  | Eighth-Rate 1.0 kbps                       |        20       |
 | 7  | (reserved)                                 |         -       |
 | 8  | (reserved)                                 |         -       |
 | 9  | CNG (AMR-WB SID)                           |        40       |
 | 10 | (reserved)                                 |         -       |
 | 11 | (reserved)                                 |         -       |
 | 12 | (reserved)                                 |         -       |
 | 13 | (reserved)                                 |         -       |
 | 14 | Erasure (AMR-WB SPEECH_LOST)               |         0       |
 | 15 | Blank (AMR-WB NO_DATA)                     |         0       |
 +----+--------------------------------------------+-----------------+
    Table 3: VMR-WB payload frame types for real-time transport
 For multi-channel sessions, the ToC entries of all frames from a
 frame-block are placed in the ToC in consecutive order.  Therefore,
 with N channels and K speech frame-blocks in a packet, there MUST be
 N*K entries in the ToC, and the first N entries will be from the
 first frame-block, the second N entries will be from the second
 frame-block, and so on.

6.3.4. Speech Data

 Speech data of a payload contains one or more speech frames as
 described in the ToC of the payload.
 Each speech frame represents 20 ms of speech encoded in one of the
 available encoding rates depending on the operation mode.  The length
 of the speech frame is defined by the frame type in the FT field,
 with the following considerations:
  1. The last octet of each speech frame MUST be padded with zeroes at

the end if not all bits in the octet are used. In other words,

   each speech frame MUST be octet-aligned.
  1. When multiple speech frames are present in the speech data, the

speech frames MUST be arranged one whole frame after another.

Ahmadi Standards Track [Page 20] RFC 4348 VMR-WB RTP Payload Format January 2006

 The order and numbering notation of the speech data bits are as
 specified in the VMR-WB standard specification [1].
 The payload begins with the payload header of one octet, or two if
 frame interleaving is selected.  The payload header is followed by
 the table of contents consisting of a list of one-octet ToC entries.
 The speech data follows the table of contents.  For the purpose of
 packetization, all the octets comprising a speech frame are appended
 to the payload as a unit.  The speech frames are packed in the same
 order as their corresponding ToC entries are arranged in the ToC
 list, with the exception that if a given frame has a ToC entry with
 FT=14 or 15, there will be no data octets present for that frame.

6.3.5. Payload Example: Basic Single Channel Payload Carrying Multiple

      Frames
 The following diagram shows an octet-aligned payload format from a
 single channel session that carries two VMR-WB Full-Rate frames
 (FT=3).  In the payload, a codec mode request is sent (e.g., CMR=4),
 requesting that the encoder at the receiver's side use VMR-WB mode 1.
 No interleaving is used.  Note that in the example below the last
 octet in both speech frames is padded with zeros to make them octet
 aligned.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | CMR=4 |R|R|R|R|1|FT#1=3 |Q|P|P|0|FT#2=3 |Q|P|P|   f1(0..7)    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   f1(8..15)   |  f1(16..23)   |  ...                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | r |P|P|P|P|P|P|  f2(0..7)     |   f2(8..15)   |  f2(16..23)   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : ...                                                           :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        ...    | l |P|P|P|P|P|P|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    r= f1(264,265)
    l= f2(264,265)

Ahmadi Standards Track [Page 21] RFC 4348 VMR-WB RTP Payload Format January 2006

6.4. Implementation Considerations

 An application implementing this payload format MUST understand all
 the payload parameters.  Any mapping of the parameters to a signaling
 protocol MUST support all parameters.  Therefore, an implementation
 of this payload format in an application using SDP is required to
 understand all the payload parameters in their SDP-mapped form.  This
 requirement ensures that an implementation always can decide whether
 it is capable of communicating.
 To enable efficient interoperable interconnection with AMR-WB and to
 ensure that a VMR-WB terminal appropriately declares itself as a
 AMR-WB-capable terminal (see Section 9.3), it is also RECOMMENDED
 that a VMR-WB RTP payload implementation understand relevant AMR-WB
 signaling.
 To further ensure interoperability between various implementations of
 VMR-WB, implementations SHALL support both header-free and octet-
 aligned payload formats.  Support of interleaving is optional.

6.4.1. Decoding Validation and Provision for Lost or Late Packets

 When processing a received payload packet, if the receiver finds that
 the calculated payload length, based on the information of the
 session and the values found in the payload header fields, does not
 match the size of the received packet, the receiver SHOULD discard
 the packet to avoid potential degradation of speech quality and to
 invoke the VMR-WB built-in frame error concealment mechanism.
 Therefore, invalid packets SHALL be treated as lost packets.
 Late packets (i.e., the unavailability of a packet when it is needed
 for decoding at the receiver) should be treated as lost packets.
 Furthermore, if the late packet is part of an interleave group,
 depending upon the availability of the other packets in that
 interleave group, decoding must be resumed from the next available
 frame (sequential order).  In other words, the unavailability of a
 packet in an interleave group at a certain time should not invalidate
 the other packets within that interleave group that may arrive later.

Ahmadi Standards Track [Page 22] RFC 4348 VMR-WB RTP Payload Format January 2006

7. Congestion Control

 The general congestion control considerations for transporting RTP
 data apply to VMR-WB speech over RTP as well.  However, the multimode
 capability of VMR-WB speech codec may provide an advantage over other
 payload formats for controlling congestion since the bandwidth demand
 can be adjusted by selecting a different operating mode.
 Another parameter that may impact the bandwidth demand for VMR-WB is
 the number of frame-blocks that are encapsulated in each RTP payload.
 Packing more frame-blocks in each RTP payload can reduce the number
 of packets sent and hence the overhead from RTP/UDP/IP headers, at
 the expense of increased delay.
 If forward error correction (FEC) is used to alleviate the packet
 loss, the amount of redundancy added by FEC will need to be regulated
 so that the use of FEC itself does not cause a congestion problem.
 Congestion control for RTP SHALL be used in accordance with RFC 3550
 [3] and any applicable RTP profile, for example, RFC 3551 [6].  This
 means that congestion control is required for any transmission over
 unmanaged best-effort networks.
 Congestion on the IP network is managed by the IP sender.  Feedback
 about congestion SHOULD be provided to that IP sender through RTCP or
 other means, and then the sender can choose to avoid congestion using
 the most appropriate mechanism.  That may include selecting an
 appropriate operating mode, but also includes adjusting the level of
 redundancy or number of frames per packet.

8. Security Considerations

 RTP packets using the payload format defined in this specification
 are subject to the general security considerations discussed in RTP
 [3] and any applicable profile such as AVP [9] or SAVP [10].
 As this format transports encoded audio, the main security issues
 include confidentiality, integrity protection, and data origin
 authentication of the audio itself.  The payload format itself does
 not have any built-in security mechanisms.  Any suitable external
 mechanisms, such as SRTP [10], MAY be used.
 This payload format and the VMR-WB decoder do not exhibit any
 significant non-uniformity in the receiver-side computational
 complexity for packet processing; thus, they are unlikely to pose a
 denial-of-service threat due to the receipt of pathological data.

Ahmadi Standards Track [Page 23] RFC 4348 VMR-WB RTP Payload Format January 2006

8.1. Confidentiality

 In order to ensure confidentiality of the encoded audio, all audio
 data bits MUST be encrypted.  There is less need to encrypt the
 payload header or the table of contents since they only carry
 information about the frame type.  This information could also be
 useful to a third party, for example, for quality monitoring.
 The use of interleaving in conjunction with encryption can have a
 negative impact on the confidentiality for a short period of time.
 Consider the following packets (in brackets) containing frame numbers
 as indicated: {10, 14, 18}, {13, 17, 21}, {16, 20, 24} (a typical
 continuous diagonal interleaving pattern).  The originator wishes to
 deny some participants the ability to hear material starting at time
 16.  Simply changing the key on the packet with the timestamp at or
 after 16, and denying the new key to those participants, does not
 achieve this; frames 17, 18, and 21 have been supplied in prior
 packets under the prior key, and error concealment may make the audio
 intelligible at least as far as frame 18 or 19, and possibly further.

8.2. Authentication and Integrity

 To authenticate the sender of the speech, an external mechanism MUST
 be used.  It is RECOMMENDED that such a mechanism protects both the
 complete RTP header and the payload (speech and data bits).
 Data tampering by a man-in-the-middle attacker could replace audio
 content and also result in erroneous depacketization/decoding that
 could lower the audio quality.  For example, tampering with the CMR
 field may result in speech of a different quality than desired.

9. Payload Format Parameters

 This section defines the parameters that may be used to select
 optional features in the VMR-WB RTP payload formats.
 The parameters are defined here as part of the MIME subtype
 registration for the VMR-WB speech codec.  A mapping of the
 parameters into the Session Description Protocol (SDP) [5] is also
 provided for those applications that use SDP.  In control protocols
 that do not use MIME or SDP, the media type parameters must be mapped
 to the appropriate format used with that control protocol.

Ahmadi Standards Track [Page 24] RFC 4348 VMR-WB RTP Payload Format January 2006

9.1. VMR-WB RTP Payload MIME Registration

 The MIME subtype for the Variable-Rate Multimode Wideband (VMR-WB)
 audio codec is allocated from the IETF tree since VMR-WB is expected
 to be a widely used speech codec in multimedia streaming and
 messaging as well as in VoIP applications.  This MIME registration
 only covers real-time transfers via RTP.
 Note, the receiver MUST ignore any unspecified parameter and use the
 default values instead.  Also note that if no input parameters are
 defined, the default values will be used.
   Media Type name:      audio
   Media subtype name:   VMR-WB
   Required parameters:  none
 Furthermore, if the interleaving parameter is present, the parameter
 "octet-align=1" MUST also be present.

OPTIONAL parameters:

mode-set:       Requested VMR-WB operating mode set.  Restricts
                the active operating modes to a subset of all
                modes.  Possible values are a comma-separated
                list of integer values.  Currently, this list
                includes modes 0, 1, 2, and 3 [1], but MAY be
                extended in the future.  If such mode-set is
                specified during session initiation, the encoder
                MUST NOT use modes outside of the subset.  If not
                present, all operating modes in the set 0 to 3 are
                allowed for the session.
channels:       The number of audio channels.  The possible
                values and their respective channel order
                is specified in Section 4.1 in [6].  If
                omitted, it has the default value of 1.
octet-align:    RTP payload format; permissible values are 0 and
                1.  If 1, octet-aligned payload format SHALL be
                used.  If 0 or if not present, header-free payload
                format is employed (default).
maxptime:       See RFC 3267 [4]

Ahmadi Standards Track [Page 25] RFC 4348 VMR-WB RTP Payload Format January 2006

interleaving:   Indicates that frame-block level
                interleaving SHALL be used for the session.
                Its value defines the maximum number of
                frame-blocks allowed in an interleaving
                group (see Section 6.3.1).  If this
                parameter is not present, interleaving
                SHALL NOT be used.  The presence of this
                parameter also implies automatically that
                octet-aligned operation SHALL be used.
ptime:          See RFC2327 [5].  It SHALL be at least one
                frame size for VMR-WB.
dtx:            Permissible values are 0 and 1.  The default
                is 0 (i.e., No DTX) where VMR-WB normally
                operates as a continuous variable-rate
                codec.  If dtx=1, the VMR-WB codec will
                operate in discontinuous transmission mode
                where silence descriptor (SID) frames are
                sent by the VMR-WB encoder during silence
                intervals with an adjustable update
                frequency.  The selection of the SID update-rate
                depends on the implementation and
                other network considerations that are
                beyond the scope of this specification.
 Encoding considerations:
        This type is only defined for transfer of VMR-WB-encoded data
        via RTP (RFC 3550) using the payload formats specified in
        Section 6 of RFC 4348.
 Security considerations:
        See Section 8 of RFC 4348.
 Public specification:
        The VMR-WB speech codec is specified in
        3GPP2 specifications C.S0052-0 version 1.0.
        Transfer methods are specified in RFC 4348.
 Additional information:
 Person & email address to contact for further information:
        Sassan Ahmadi, Ph.D.        sassan.ahmadi@ieee.org

Ahmadi Standards Track [Page 26] RFC 4348 VMR-WB RTP Payload Format January 2006

 Intended usage: COMMON.
   It is expected that many VoIP, multimedia messaging and
   streaming applications (as well as mobile applications)
   will use this type.
 Author/Change controller:
   IETF Audio/Video Transport working group delegated from the IESG

9.2. Mapping MIME Parameters into SDP

 The information carried in the MIME media type specification has a
 specific mapping to fields in the Session Description Protocol (SDP)
 [5], which is commonly used to describe RTP sessions.  When SDP is
 used to specify sessions employing the VMR-WB codec, the mapping is
 as follows:
  1. The media type ("audio") goes in SDP "m=" as the media name.
  1. The media subtype (payload format name) goes in SDP "a=rtpmap"

as the encoding name. The RTP clock rate in "a=rtpmap" MUST be

      16000 for VMR-WB.
  1. The parameter "channels" (number of channels) MUST be either

explicitly set to N or omitted, implying a default value of 1.

      The values of N that are allowed is specified in Section 4.1 in
      [6].  The parameter "channels", if present, is specified
      subsequent to the MIME subtype and RTP clock rate as an encoding
      parameter in the "a=rtpmap" attribute.
  1. The parameters "ptime" and "maxptime" go in the SDP "a=ptime"

and

         "a=maxptime" attributes, respectively.
  1. Any remaining parameters go in the SDP "a=fmtp" attribute by

copying them directly from the MIME media type string as a

      semicolon-separated list of parameter=value pairs.
 Some examples of SDP session descriptions utilizing VMR-WB encodings
 follow.
 Example of usage of VMR-WB in a possible VoIP scenario (wideband
 audio):
    m=audio 49120 RTP/AVP 98
    a=rtpmap:98 VMR-WB/16000
    a=fmtp:98 octet-align=1

Ahmadi Standards Track [Page 27] RFC 4348 VMR-WB RTP Payload Format January 2006

 Example of usage of VMR-WB in a possible streaming scenario (two
 channel stereo):
    m=audio 49120 RTP/AVP 99
    a=rtpmap:99 VMR-WB/16000/2
    a=fmtp:99 octet-align=1; interleaving=30
    a=maxptime:100

9.3. Offer-Answer Model Considerations

 To achieve good interoperability for the VMR-WB RTP payload in an
 Offer-Answer negotiation usage in SDP [13], the following
 considerations are made:
  1. The rate, channel, and payload configuration parameters (octet-

align and interleaving) SHALL be used symmetrically, i.e., offer

   and answer must use the same values.  The maximum size of the
   interleaving buffer is, however, declarative, and each agent
   specifies the value it supports to receive for recvonly and
   sendrecv streams.  For sendonly streams, the value indicates what
   the agent desires to use.
  1. To maintain interoperability among all implementations of VMR-WB

that may or may not support all the codec's modes of operation, the

   operational modes that are supported by an implementation MAY be
   identified at session initiation.  The mode-set parameter is
   declarative, and only operating modes that have been indicated to
   be supported by both ends SHALL be used.  If the answerer is not
   supporting any of the operating modes provided in the offer, the
   complete payload type declaration SHOULD be rejected by removing it
   from the answer.
  1. The remaining parameters are all declarative; i.e., for sendonly

streams they provide parameters that the agent desires to use,

   while for recvonly and sendrecv streams they declare the parameters
   that it accepts to receive.  The dtx parameter is used to indicate
   DTX support and capability, while the media sender is only
   RECOMMENDED to send using the DTX in these cases.  If DTX is not
   supported by the media sender, it will send media without DTX; this
   will not affect interoperability only the resource consumption.
  1. Both header-free and octet-aligned payload format configurations

MAY be offered by a VMR-WB enabled terminal. However, for an

   interoperable interconnection with AMR-WB, only octet-aligned
  1. The parameters "maxptime" and "ptime" should in most cases not

affect the interoperability; however, the setting of the parameters

   can affect the performance of the application.

Ahmadi Standards Track [Page 28] RFC 4348 VMR-WB RTP Payload Format January 2006

  1. To maintain interoperability with AMR-WB in cases where negotiation

is possible using the VMR-WB interoperable mode, a VMR-WB-enabled

   terminal SHOULD also declare itself capable of AMR-WB with limited
   mode set (i.e., only AMR-WB codec modes 0, 1, and 2 are allowed)
   and of octet-align mode of operation.
 Example:
              m=audio 49120 RTP/AVP 98 99
              a=rtpmap:98 VMR-WB/16000
              a=rtpmap:99 AMR-WB/16000
              a=fmtp:99 octet-align=1; mode-set=0,1,2
 An example of offer-answer exchange for the VoIP scenario described
 in Section 5.3 is as follows:
     CDMA2000 terminal -> WCDMA terminal Offer:
              m=audio 49120 RTP/AVP 98 97
              a=rtpmap:98 VMR-WB/16000
              a=fmtp:98 octet-align=1
              a=rtpmap:97 AMR-WB/16000
              a=fmtp:97 mode-set=0,1,2; octet-align=1
     WCDMA terminal -> CDMA2000 terminal Answer:
              m=audio 49120 RTP/AVP 97
              a=rtpmap:97 AMR-WB/16000
              a=fmtp:97 mode-set=0,1,2; octet-align=1;
 For declarative use of SDP such as in SAP [14] and RTSP [15], all
 parameters are declarative and provide the parameters that SHALL be
 used when receiving and/or sending the configured stream.

10. IANA Considerations

 The IANA has registered one new MIME subtype (audio/VMR-WB); see
 Section 9.

11. Acknowledgements

 The author would like to thank Redwan Salami of VoiceAge Corporation,
 Ari Lakaniemi of Nokia Inc., and IETF/AVT chairs Colin Perkins and
 Magnus Westerlund for their technical comments to improve this
 document.
 Also, the author would like to acknowledge that some parts of RFC
 3267 [4] and RFC 3558 [11] have been used in this document.

Ahmadi Standards Track [Page 29] RFC 4348 VMR-WB RTP Payload Format January 2006

12. References

12.1. Normative References

 [1]  3GPP2 C.S0052-0 v1.0 "Source-Controlled Variable-Rate Multimode
      Wideband Speech Codec (VMR-WB) Service Option 62 for Spread
      Spectrum Systems", 3GPP2 Technical Specification, July 2004.
 [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [3]  Schulzrinne, H.,  Casner, S., Frederick, R., and V. Jacobson,
      "RTP: A Transport Protocol for Real-Time Applications", STD 64,
      RFC 3550, July 2003.
 [4]  Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "Real-
      Time Transport Protocol (RTP) Payload Format and File Storage
      Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate
      Wideband (AMR-WB) Audio Codecs", RFC 3267, June 2002.
 [5]  Handley, M. and V. Jacobson, "SDP: Session Description
      Protocol", RFC 2327, April 1998.
 [6]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
      Conferences with Minimal Control", STD 65, RFC 3551, July 2003.

12.2. Informative References

 [7]  3GPP2 C.S0050-A v1.0 "3GPP2 File Formats for Multimedia
      Services", 3GPP2 Technical Specification, September 2005.
 [8]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
      Generic Forward Error Correction", RFC 2733, December 1999.
 [9]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
      Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
      3711, March 2004.
 [10] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M.,
      Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP Payload
      for Redundant Audio Data", RFC 2198, September 1997.
 [11] Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs
      (EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558, July 2003.
 [12] 3GPP TS 26.193 "AMR Wideband Speech Codec; Source Controlled
      Rate operation", version 5.0.0 (2001-03), 3rd Generation
      Partnership Project (3GPP).

Ahmadi Standards Track [Page 30] RFC 4348 VMR-WB RTP Payload Format January 2006

 [13] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
      Session Description Protocol (SDP)", RFC 3264, June 2002.
 [14] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
      Protocol", RFC 2974, October 2000.
 [15] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
      Protocol (RTSP)", RFC 2326, April 1998.
 Any 3GPP2 document can be downloaded from the 3GPP2 web server,
 "http://www.3gpp2.org/", see specifications.

Author's Address

 Dr. Sassan Ahmadi
 EMail: sassan.ahmadi@ieee.org

Ahmadi Standards Track [Page 31] RFC 4348 VMR-WB RTP Payload Format January 2006

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Ahmadi Standards Track [Page 32]

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